'70 C10 Temp Gauge Max Hot, Why?
 

1970 C10, 402 engine, with gauges. Temp gauge was max cold when I got the truck last summer.

I put in a new temp sensor, new OEM temp gauge, new instrument panel bulbs and bulb holders, and new instrument panel printed circuit board. Now temp is always max hot. I did continuity check on wire from temp sensor to passenger-side post on temp gauge and it checked good. Got 12V on drivers side post of temp gauge.

I'm stumped. Any ideas?

Re: '70 C10 Temp Gauge Max Hot, Why?
 

There are two senders for our trucks. One is for gauges and one is for lights. Are you sure you got the right one? I will see if I can find pictures of the difference.

LockDoc

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Your sender wire is grounded someplace, or the sender is no good. Check the terminal where the wire goes on the sender to the sender body with an ohmmeter for around 200 ohms cold. Take the wire off the sender first to see if the gauge goes to cold.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

just because a part is new does not mean it will work, new parts are bad all the time, the sender could be bad.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Here are some pics. Temp sensor is on an Edelbrock RPM Airgap intake manifold.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

with the temp sensor wire off the sensor the temp gauge stays at max hot, ignition switch on and off.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

with one lead on temp sensor top and the other lead on temp sensor base, ohms are 171, engine is mildly warm still.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

So it appears I've got a ground somewhere. What puzzles me is I checked continuity from temp sensor wire end to the gauge and continuity checked good. Could that happen if there was a ground somewhere?

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Shouldn't your sending unit be on the side of the DS head? On a related note, where can a guy get the connector for the sending unit?

Re: '70 C10 Temp Gauge Max Hot, Why?
 

My instrument panel connector matches this schematic

Re: '70 C10 Temp Gauge Max Hot, Why?
 

https://www.summitracing.com/parts/PCO-5665PT

for the sending unit wire connector. No one in Las Vegas had them. It's on order, haven't received it yet, Summit told me that is what fits.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

The gauge itself could be hosed. That's what happened to mine in identical circumstances. I doubt that there is a short in the wire, since the sender for a gauge is a thermopile that sends power to the meter. A short would leave the gauge indicating a lower temperature, as less power would be seen by the gauge.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

If your gauge reads full hot with the key off then your gauge May be bad. If your sender wire is pinched and grounded, it will read continuity end to end even through the ground.
you should unplug the cluster plug and if the wire is shorted to ground then the gauge will return to cold unless the short is in the cluster body. Some of these gauges will stay the same when the power is off as they read while the key was on. The needle on the temp gauge would stay where it's at until the ground is removed. That would explain why it reads past hot all the time.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Old thread but my temp gauge is pegged just like in post 5.

I have all new wiring harnesses, gauges, printed circuit, "everything." With temp sending unit wire disconnected and battery disconnected, still pegged.
From what I am reading, gauge must be bad???

Sucks, if that's the case because pulling the cluster is bummer as stuff always ends up worse (in my experience). After putting the whole truck together, only this gauge and dome light don't work so I guess I am lucky (for now).

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Did it ever register anything else? I would hook it all up, turn the key on and rap on the cluster in that area (lightly) with the side of my fist to see if it is just stuck.
.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

It was on cold when I installed the dash and set that way for months. After installing all the wiring and hooking up the battery, I noticed it was on max when I did my firing test.

As suggested, I went out the the truck with the intent of giving the gauge a litlle bang with my knuckle from the backside.

The battery and sending unit still disconnected, reached under the dash and touched the temp terminals with my fingers and heard "spring/click" sound. The needle popped back to the cold side. ????? Go figure. Me being a ground??? Luck?? Didn't even tap it.

Will try hooking everything up in the morning and fire it up to see what happens. But for now it is on the cold side where it belongs. A victory for now.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Had a similar thing happen once after I changed the cab on a 72 pickup.
Temp gage would go full hot when key was in the start position.
Problem went away after I installed the ground straps from engine to cab and from cab to frame.
Was the gage carrying the full ground to the cab? I don't know.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Hooked the sending unit wire back up, the battery, turned the key to on and quickly pegged the needle again. Reversed what I had done but this time touching the back of the terminal and tapping on it had no effect. Still pegged.

Bad ground seems likely but everything else on the cluster works.
I have two ground straps to the cab already, as I know to use lots of grounding points on these trucks. Because the dome light doesn't work also, I will take off and re-tighten the ground straps to see what happens.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

In other GM forums, guys have had issues with the currently available sending units, as the resistance values are not the same as original. Only AC NOS or Lectric Limited reproductions are correct to get a somewhat accurate reading.

https://www.lectriclimited.com/catal.../category/734/

Re: '70 C10 Temp Gauge Max Hot, Why?
 

The new one I installed was just over the counter at Napa. So that could be the issue. So many variables.........

Re: '70 C10 Temp Gauge Max Hot, Why?
 

The circuit works so when the wire is off of the sensor the guage should read low. When the wire to sensor is grounded to the block the guage should peg high and it should be some where in between when hooked to the sensor. The key must be on for these tests.

If you get no action then I would check the continuity of the wire from the sensor to the guage plug at the circuit board disconnect the sensor from the wire for this. If you have good continuity then I would pull the guage. It is possible to just remove the temperature guage pair from the rest of the guages. Possible not easy.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Cold tested per above .806

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Hmmm I took off both nuts and washers from the printed circuit to gauge terminals and the gauge popped back to cold. I put it back together, watching the needle as I slowly turned the nuts back on. Had the key on while doing it. Gauge started working perfectly but as I turned the nut a bit tighter, pegged hot. So I loosened the nut (barely snug) and it now works as it should. Although something is not right, it works. Would be hard to believe the nut would have to be sloppy loose to work.
Should the lock washer and nut be right against the printed circuit like in photo?

Thanks for the help. Enough to drive a guy crazy

Re: '70 C10 Temp Gauge Max Hot, Why?
 

It could be that the gauge is attached crooked or off-center on the mounting plate which could cause one of the posts to short out on the mounting plate. I've seen that issue before.

Re: '70 C10 Temp Gauge Max Hot, Why?
 

Thanks. Amazing how many different thinks can go wrong or possibly wrong with one system or piece.